1. Field of the Invention
This invention relates to a digital amplitude modulation amplifier, and more particularly to a digital amplitude modulation amplifier which converts a modulation signal into a digital signal, power amplifies a transmission carrier in accordance with the digital signal and outputs an amplitude modulated wave.
The present invention relates also to a television broadcasting machine, and more particularly to a television broadcasting machine which can transmit a vestigial side band modulated (VSB) television broadcasting signal by phase modulating and amplitude modulating a carrier with phase information and amplitude information obtained from a video signal.
2. Description of the Related Art
FIG. 1 shows a block diagram of an example of a conventional amplitude modulation amplifier. Referring to FIG. 1, a carrier outputted from carrier generator 11 and a modulation signal outputted from modulation signal generator 12 are supplied to amplitude modulator 41, by which an amplitude modulated wave is produced by modulating the amplitude of the carrier with the modulation signal. The amplitude modulated wave is supplied to and power amplified by linear power amplifier 42 which performs an A class or AB class operation. Consequently, the carrier (amplitude modulated wave) whose amplitude is modulated in accordance with the modulation signal is outputted from linear power amplifier 42.
FIG. 2 shows a block diagram of an example of a conventional digital amplitude modulation amplifier. This conventional digital amplitude modulation amplifier is disclosed in U.S. Pat. No. 4,804,931 and can control the output level thereof with a digital signal in the form of a modulation signal.
Referring to FIG. 2, an analog modulation signal generated from modulation signal generator 12 is supplied to A/D converter 13, by which it is converted into a digital modulation signal. The digital modulation signal is supplied to on/off controller 14, by which it is converted into a control signal for on/off switching four switches SW1 to SW4. Here, since four switches SW1 to SW4 are involved, the quantization bit number of the output digital signal of A/D converter 13 described above is 4 bits, and the control signal mentioned above on/off switches switches SW1 to SW4 corresponding to the individual bits.
Switches SW1 to SW4 receive a carrier generated by carrier generator 11 as input signals thereto and supply the inputted carriers to power amplifiers
to PA4 provided corresponding thereto for periods for which they are controlled to an on state. Since a linear characteristic is not required for power amplifiers PA1 to PA4, they are constructed so as to effect a C class operation in which they exhibit a high power utilization efficiency. Further, power amplifiers PA1 to PA4 have amplification degrees weighted for the individual bits of the digital signal of 4 bits. For example, power amplifier PA1 has an amplification degree which increases in proportion to 23 corresponding to the most significant bit (MSB) of the 4-bit digital signal, and similarly, power amplifiers PA2, PA3 and PA4 have amplification degrees which increase in proportion to 22, 21 and 20, respectively.
An output signal of each of power amplifiers PA1 to PA4 is inputted to the one of the two input signals of the corresponding one of the 3 dB couplers CO1 to CO4 provided corresponding to power amplifiers PA1 to PA4 and is coupled to another input signal to the other input terminal. Here, output signals of the 3 dB couplers CO2, CO3 and CO4 of the preceding bits (lower bits) are inputted to the other input terminals of the 3 dB couplers CO1, CO2 and CO3. Further, the carrier from carrier generator 11 amplified by initial signal amplifier 19 is inputted to the other input terminal of the 3 dB coupler CO4 of the least significant bit (LSB). It is to be noted that dummy resistors R1 or R4 are connected to dummy outputs of 3 dB couplers CO0 to CO4, respectively.
3 dB couplers CO1 to CO4 have the same construction, and as a representative of them, the construction of coupler CO1 is shown in FIG. 3. Referring to FIG. 3, coupler CO1 has input terminal 51 to which an output signal of coupler CO2 of the lower bit is inputted, input terminal 52 to which an output signal of power amplifier PA1 of the self bit is inputted, output terminal 53 from which a coupling output signal is outputted, and dummy output terminal 54.
Since the output signal of coupler CO2 of the lower bit is a coupling output signal of 3 dB couplers CO4, CO3 and CO2 (in other words, a composite signal of the output signals of power amplifiers PA4, PA3 and
in accordance with on/off states of switches SW4, SW3 and SW2), as can be seen from the construction of FIG. 3, an output signal composed in accordance with on/off states of switches SW1 to SW4 from among the output signals of power amplifiers PA1 to PA4 is outputted from output terminal 53 of 3 dB coupler CO1. In particular, the output signal of 3 dB coupler CO1 in this instance is an amplitude modulated wave varied in accordance with on/off states of switches SW1 to SW4 and having an amplitude modulated in accordance with the modulation signal. FIG. 4 is a diagrammatic view of a concept of composition of the amplitude modulated wave, and reference symbols pa1 to pa4 denote output signal components of power amplifiers PA1 to PA4 of the output amplitude modulated wave.
The conventional amplitude modulation amplifier shown in FIG. 1, however, has a problem in that the power utilization efficiency is low because an amplitude modulated signal is power amplified by linear power amplifier 42 which effects an A class or AB class operation. This is described with reference to FIGS. 5(A) and 5(B). An amplitude modulated wave exhibits a large difference between a peak level and an average level of the signal as seen from FIG. 5(B). In order to amplify an amplitude modulated wave having such character as just described with minimized distortion, a linear power amplifier which effects an A class or AB class operation is used, and the efficiency of the power amplifier of the type just mentioned drops in accordance with a drop of the output level as seen from FIG. 5(A). Therefore, the efficiency at an average output level exhibits a value lower than a peak efficiency.
Meanwhile, the conventional digital amplitude modulation amplifier shown in FIG. 2 has such problems as described below since it employs 3 dB couplers CO1 to CO4. In particular, if voltage E1 inputted to input terminal 51 and voltage E2 inputted to input terminal 52 in FIG. 3 have a relationship such that the phases thereof may be 0 degree and 90 degrees, respectively, output voltage EOUT of output terminal 53 and dummy voltage EDUM of dummy output terminal 54 have a relationship such as the ones represented by the following expressions: ##EQU1##
According to the expressions above, when E1=E2, dummy voltage EDUM to dummy output terminal 54 is 0, and the input voltages are all led out to output terminal 53. However, input voltage E1 to input terminal 51 is a composite output signal of power amplifiers PA2 to PA4 of the lower bits and varies in accordance with on/off states of switches SW2 to SW4 on the input sides of power amplifiers PA2 to PA4 (that is, the values of the 2 to 4 bits of the digital signal). Also input signal E2 to input terminal 52 varies between presence and absence depending upon the state of its corresponding bit.
Accordingly, depending upon the on/off states of switches SW1 to SW4 on the input sides of power amplifiers PA1 to PA4 (that is, the values of the bits of the digital signal), all of the signal power inputted to 3 dB coupler CO1 is not led out to output terminal 53, but part of the signal power is absorbed from dummy output terminal 54 by dummy resistor R1, resulting in drop of the power utilization efficiency. Also the modulation degree of the output signal from output terminal 53 does not exhibit regeneration of the amplitude of the input modulation degree with fidelity.
Meanwhile, a conventional television broadcasting machine includes, for example, as shown in a block diagram of FIG. 6, input terminal 101 for receiving an analog video signal, signal invertor 103, amplitude modulator 150, carrier generator 151, vestigial side band filter (VSBF) 152, power amplifier 153 and output terminal 117.
Operation of the conventional television broadcasting machine is described below. An analog video signal inputted through input terminal 101 is inverted by signal invertor 103 and then inputted as a modulation signal to amplitude modulator 150, by which a carrier from carrier generator 151 is amplitude modulated with the modulation signal to form an amplitude modulated wave. Since the amplitude modulated wave is a double side band (DSB) signal, it is supplied to vestigial side band filter (VSBF) 152, by which most of a lower side band is removed therefrom to form a vestigial side band (VSB) signal. Then, the vestigial side band signal is power amplified by power amplifier 153 and outputted to output terminal 117 so that it may be multiplexed and transmitted with an audio modulated wave.
However, since the conventional television broadcasting machine requires vestigial side band filter 152 irrespective of whether it is of a lower power stage or a high power stage and vestigial side band filter 152 is a part so important that it decides a characteristic of the entire broadcasting machine and is required to be formed with a particularly high degree of accuracy, it has a problem in that vestigial side band filter 152 is a special filter and is expensive.